When a living tissue is placed under uniform magnetostatic field (hereinafter referred to as BO. In this description, a BO direction is defined as z direction) in an MRI device, the magnetic moment of atomic nuclear spins of tissue composition molecules makes a precession movement around the BO direction at a resonance frequency inherent to each spin. When these spins are exposed to magnetic field (irradiation radio-frequency magnetic field B1) having a frequency near to the resonance frequency from a direction perpendicular to the BO direction, a net magnetic moment M is rotated (excited) toward x-y plane, a net transverse magnetic moment occurs. Thereafter, when the irradiation radio-frequency magnetic field B1 is turned off, the excited magnetic moment is returned (relaxed) to the original state while emitting energy (NMR signal). At this time, the MRI device detects the emitted NMR signal (echo signal), and subjects the NMR signal to signal processing to obtain an image of a living tissue (actual imaging).
In this case, there is a technique of applying a pre-pulse having specific radio-frequency magnetic field before actual imaging to bring the emitted echo signal with some effect. For example, there is known a technique (CHESS method) for applying a pre-pulse called as a CHESS (Chemical Shift Suppression) pulse to suppress an echo signal from hydrogen protons of fat molecules (hereinafter abbreviated as “fat proton”) and imaging only a signal from water molecules (for example, see non-patent document 1). According to the CHESS method, a radio-frequency magnetic field (hereinafter abbreviated as “RF”) pulse of a fixed magnetic intensity (in this case, a flip angle is 90°) having a resonance frequency of fat protons is applied as a CHESS pulse to a living body to selectively excite only the fat protons before actual imaging, and then a crusher gradient magnetic field pulse is applied. Accordingly, transverse magnetization of the fat protons which are selectively excited by the CHESS pulse are subjected to phase dispersion to vanish the magnetization of the fat protons just before the actual imaging, thereby suppressing the signal from the fat protons.
In general, the effect of the pre-pulse is reduced with time lapse. For example, according to the CHESS method, the longitudinal magnetization of fat protons recovers on the basis of the longitudinal relaxation time of the fat protons as the time elapses just after the CHESS pulse is applied. Therefore, a signal collected just after the CHESS pulse is applied have a fat suppression effect, and the fat suppression effect is reduced as the time elapses from the application of the CHESS pulse.
Multi-slice imaging using the pre-pulse as described above is performed. For example, FIG. 12(a) is a sequence diagram in the multi-slice imaging using a CHESS pulse. Here, only a pulse sequence on the RF axis is shown. In the multi-slice imaging, a signal of one slice is generally obtained during a repeat period TR for another slice, thereby shortening the imaging time. At this time, in order to obtain a sufficient CHESS pulse effect for each slice, a CHESS pulse 201 is applied just before each exciting pulse 202 of actual imaging is applied as shown in FIG. 12(a). A number affixed to each exciting pulse 202 represents a slice number representing a slice order of slice for a measurement target. In this example, N slices of slice numbers from 1 to N are successively imaged in this order.
FIG. 12(b) is a schematic diagram showing a repeat loop of the multi-slice imaging, and an inner loop corresponds to a subordinate loop. As shown in FIG. 12(b), the measurement is executed with the amount of one phase encode for the number of times corresponding to the number of slices, and is repeated for the number of required phase encode steps while varying the phase encode amount.